1. Field of the Invention
The present invention relates to a scanning optical system, and particularly to a scanning optical system of a mirror scanning type as used, for example, in a film scanner for high-speed image reading.
2. Description of the Prior Art
Of various types of scanning optical system conventionally proposed, those adopting mirror scanning are the most effective types in high-speed and high-resolution image reading (refer to Japanese Patent Applications Laid-Open Nos. H9-236741, H9-236747, H9-236766, and H9-236767). A typical mirror-scanning-type scanning optical system is composed of an image-forming optical system for forming an image of a object on a one-dimensional line sensor (for example, an image-sensing device such as a line CCD (charge-coupled device)) having light-sensing elements arranged in a line and a mirror that is rotated to swing in such a way as to scan the object in the sub scanning direction. The image-forming optical system is composed of an object-side lens unit and an image-side lens unit. The light from the object is directed through the object-side lens unit to the mirror so as to be deflected thereby, and is then focused through the image-side lens unit on the one-dimensional line sensor so as to form an image thereon. In the present specification, the direction in which the light-sensing elements of the one-dimensional line sensor are arranged is defined as the "main scanning direction", and the direction in which the object is scanned by the rotation of the mirror is defined as the "sub scanning direction".
A mirror-scanning-type scanning optical system has the disadvantage that, as its mirror is rotated to scan the object, various aberrations, such as chromatic aberration and coma, appear according to the angle at which the mirror is swung. Increasing the number of constituent lens elements to correct those aberrations leads to an undue increase in the costs of the scanning optical system.
On the other hand, reading of color images is often realized by the use of color-separation prisms. However, to minimize the costs of a scanning optical system, it is effective to eliminate the use of color-separation prisms, which are expensive. For example, using a trilinear image-sensing device makes it possible to read color images at high speed and with high resolution without color-separation prisms. However, a trilinear image-sensing device has three one-dimensional line sensors fixed parallel to one another in the sub scanning direction, and therefore its use necessitates, in addition to correction of lateral chromatic aberration, correction of longitudinal chromatic aberration, which does not pose any serious problem in a scanning optical system employing color-separation prisms. This makes it necessary to further enhance the optical performance of the scanning optical system, and thus to increase the number of constituent lens elements of the scanning optical system.